Membrane filtration system with concentrate staging and concentrate recirculation, switchable stages, or both

ABSTRACT

A membrane filtration system with reverse osmosis (RO) or nanofiltration (NF) elements is adapted to provide high recovery from difficult wastewater. The system has a plurality of stages. The system is configured to provide concentrate staging. The last stage also has concentrate recirculation. The valves and pumps of the system are arranged such that the order of flow and a recirculation pump may be switched between the first stage and the last stage at some times.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/775,821, filed Sep. 14, 2015, which is a National Stage Entry ofInternational Application No. PCT/CN2013/072588, filed Mar. 14, 2013.

FIELD

This specification relates to membrane filtration, for example reverseosmosis or nanofiltration.

BACKGROUND

Reverse osmosis (RO) and nanofiltration (NF) membranes are typicallyused in the form of elements, also called modules, such as spiral woundelements, hollow fiber elements or tubular elements. A number ofelements, typically between 1 and 8, are mounted in series in a pressurevessel, alternatively called a housing, with a feed inlet, concentrateoutlet, and permeate outlet. Multiple pressure vessels may be connectedtogether in parallel to form a bank, alternatively called a stage, in afiltration system. The stages of a filtration system are collectivelyreferred to as a membrane block.

A filtration system may have multiple stages connected together invarious configurations. In concentrate staging, alternatively called amulti-stage array, feed water is first pumped into a first stage ofelements. Concentrate from each upstream stage is fed to each downstreamstage. The concentrate port of the last stage is fitted with aconcentrate valve. The flow and pressure through the membrane block arecontrolled by the feed pump and concentrate valve. Permeate flows fromeach stage to a common permeate header. The concentrate stagingincreases permeate recovery. Filtration systems with high recoveryrates, for example 80% or more, typically have at least two stages.

BRIEF SUMMARY OF THE INVENTION

Although concentrate staging increases the recovery rate of a system,the flow rate of concentrate declines in each stage. With some types ofwastewater, the flow rate in a high recovery filtration system may beinsufficient to prevent fouling in the last stage.

In a filtration system described in this specification, two or morestages are connected together so as to provide concentrate staging. Thelast stage has a recirculation pump and conduits configured to provideconcentrate recirculation. In another filtration system, valves andconduits of the system are arranged to allow the order in which feedwater flows through a portion of a first and a last stage to beswitched. The portion may be the entire first stage or less than theentire first stage. In an embodiment, with or without the order of flowswitched, the system provides concentrate staging and concentraterecirculation in the stage that receives concentrate last.

In a filtration process described in this specification, feed water isseparated into permeate and a first concentrate. The first concentrateis separated into permeate and a second concentrate. Part of the secondconcentrate portion is recycled and mixed with the first concentrate. Inanother process, the order of flow is switched between a portion of afirst stage and a last stage of a filtration system at some times. Theportion may be the entire first stage or a portion that is less than theentire first stage. In an embodiment, in each order, the systemimplements a filtration process having both concentrate staging andconcentrate recirculation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic process flow diagram of a filtration system.

FIG. 2 is a schematic process flow diagram of a second filtrationsystem.

FIG. 3 is a schematic process flow diagram of a third filtration system.

DETAILED DESCRIPTION

Referring to FIG. 1, a filtration system 10 treats feed water 12 toproduce effluent 14. The filtration system 10 has a feed pump 22, afirst stage 16, a last stage 20, a recirculation pump 24 and brine 26.In an embodiment, the system 10 also has one or more intermediate stages18. In the system 10, the last stage 20 may be called the third stagealthough in other systems the last stage 20 could be the second stage,fourth stage or another stage. Also in the system 10, the intermediatestage 18 may be called the second stage although in other systemsintermediate stages could include second, third, fourth or more stages.

Each stage comprises a set of one or more membrane filtration elements,for example nanofiltration or reverse osmosis elements. In an embodimentmultiple elements in a stage are provided in series in a pressurevessel. Larger stages may comprise multiple pressure vessels plumbed inparallel, for example with a feed manifold connected to an inlet of eachof the 8 pressure vessels, a concentrate manifold connected to aconcentrate outlet of each of the 8 pressure vessels, and a permeatemanifold connected to a permeate outlet of each of the 8 pressurevessels. In an embodiment first stage 16 is larger (i.e. has more of thesame size elements) than the intermediate stage 18. In an embodimentlast stage 20 is larger than the intermediate stage 18. The first stage16 is at least as large as the last stage 20.

The filtration system also has valves V1 to V9 and various conduitsconfigured to provide the flow paths described below. Valves V1 to V9may move between fully opened and fully closed positions. However, in anembodiment, valve V9 is a throttle valve that may be set to variousintermediate positions. The flow and pressure through the membrane blockare controlled by the feed pump 22 or valve V9 or both.

In a first configuration as shown, valves V1, V3, V5 and V7 are open.Valves V2, V4, V6 and V8 are closed. Valve V9 is at least partially opento provide a bleed of brine 26 at a selected flow rate. The system 10can also be operated in a second configuration with valves V1, V3, V5and V7 closed; valves V2, V4, V6 and V8 open; and, valve V9 at leastpartially open. In both configurations, the flow rate of the brine 26 ismore particularly 20% or less of the flow rate of the feed water 22. 80%or more of the feed water 22 is recovered as permeate 14.

In the first configuration, feed pump 22 pumps feed water 12 to thefirst stage 16. First stage permeate 28 is produced and becomes part ofthe effluent 14. First stage concentrate 30 is also produced and flowsto the intermediate stage 18. First stage concentrate 30 flowing througha feed side of the intermediate stage 18 is separated into second stagepermeate 32 and second stage concentrate 34. Second stage permeate 32becomes part of the effluent 14. Second stage concentrate 34 flows tothe recirculation pump 24. The recirculation pump 24 pumps the secondstage concentrate 34 to the last stage 20. Third stage permeate 36 isproduced and becomes part of the effluent 14. Third stage concentrate 38is partially discharged as brine 26 and partially returned to the feedside of the recirculation pump 24. A portion of third stage concentrate38 is thereby recirculated to the third stage 20. In summary, the system10 operates with concentrate staging between the stages 16, 18, 20 andwith concentrate recirculation in the last stage 20.

In the second configuration, the system 10 again operates withconcentrate staging between the stages 16, 18, 20 and with concentraterecirculation in the stage receiving concentrate last. However, in thiscase the third stage 20 receives the feed water 12 first; the directionof concentrate staging is from the third stage 20 to the second stage 18to the first stage 16; and first stage 16 operates with concentraterecycle, alternatively called feed and bleed.

The recirculation pump 24 and the order of flow are switched betweenthrough the first stage 16 and last stage 20 when the valves are movedfrom the first configuration to the second configuration. Moreparticularly, however, the direction of flow through the feed sides ofthe stages 16, 18, 20 does not change when the valves are moved from thefirst configuration to the second configuration. Although some elementsand pressure vessels may be configured for reversible flow, others havecomponents such as brine seals or permeate collectors that only operatewith flow in one direction, or are optimized for flow in one direction.In systems with multiple intermediate stages, the order of flow in anembodiment as between the intermediate stages also does not change whenthe valves are moved from the first configuration to the secondconfiguration. The relative number of elements, or the piping or pumpingsystem, may be optimized for flow in one direction between multipleintermediate stages.

The use of concentrate staging allows for a high recovery rate, forexample 80% or more or between 85% and 95%. Concentrate recirculation inthe last stage increases the flow rate through the last stage to helpinhibit fouling. Overall permeate quality remains high since the firstand second stages operate at reasonable feed side concentrations. Thecost and energy consumption of the recirculation pump is limited to whatis required by the last stage. However, in some cases, the last stagemay still foul. Switching the order of flow at least at some timesallows the last stage to be flushed with feed water to help furtherinhibit, or in some cases remove, fouling. In particular, some solubleorganic compounds in difficult to treat wastewater can cause fouling inthe last stage despite the concentrate recirculation. However, exposingthe last stage to un-concentrated feed water at some times flushes theorganic fouling layer from the last stage. In the system 10, the laststage 20 may be switched with the first stage 16 and receiveun-concentrated feed water 12 for up to half of the operating time ofthe system 10.

In order to facilitate switching in an embodiment, the first stage 16and last stage 20 are the same size, or at least about the same size. Ifthe first stage 16 is materially larger than the last stage 20, then thelast stage may be switched with a portion of the first stage 16 that isless than the entire first stage 16. In this case, the portion is moreparticularly the same size, or at least about the same size, as the laststage 20. For example, a portion of the first stage 16, which may be theentire first stage 16 or less than the entire first stage, that isswitched with the last stage 20 may have a number of elements orpressure vessels, or both, that is within 25% of the correspondingnumber or numbers in the last stage 20.

The system 10 can be used to treat a variety of feed water 12. However,the system 10 is particularly adapted to providing high (80% or more)recovery from difficult to treat wastewater. The feed water 12 may have200 mg/L or more of chemical oxygen demand (COD). Difficult waste watersinclude, for example, landfill leachate, coking plant wastewater,reverse osmosis brine and cooling tower blowdown. Optionally, recoverymay be increased further by treating the brine, for example with athermal evaporator, crystallizer, zero liquid discharger (ZLD) orphysical-chemical treatment system.

EXAMPLES

In a first example, a three stage nanofiltration (NF) system 10 wasdesigned for treating an industrial effluent with greater than 200 mg/ofchemical oxygen demand (COD), 90 m3/h of permeate flow, and 90%recovery. The system 10 was arranged as shown in FIG. 1. The feed pump22 is a high pressure pump rated for 100 m3/h of output and 110 m ofhead. The recirculation pump 24 is rated for 110 m3/h of output and 40 mof head.

The system has 126 nominal 8 inch (20 cm) NF spiral wound elementsinserted into 21 pressure vessels. Each pressure vessel holds 6 elementsin series. The system has three stages 16, 18, 20. The first 16 and last20 stages each have 8 pressure vessels plumbed in parallel. Theintermediate stage 18 has 5 pressure vessels plumbed in parallel.

The first and last stages 16, 20 are identical and the recirculationpump 24, and the order of flow, can be switched between them. In onevalve configuration, as shown in FIG. 1, valves V1, V3, V5 and V7 areopen and valves V2, V4, V6 and V8 are closed. The feed water 12 ispumped to the first stage 16 by the high pressure feed pump 22, theconcentrate from the first stage 16 is fed to the intermediate stage 18,and the concentrate from the intermediate stage 18 is fed to the laststage 20 through the recirculation pump 24. In a second valveconfiguration in which the first stage 16 and the last stage 20 areswitched, valves V2, V4, V6 and V8 open and valves V1, V3, V5 and V7 areclosed. The feed water 12 was pumped to the last stage 20 by the highpressure pump, the concentrate from the third stage 20 is fed to theintermediate stage 18 and the concentrate from the intermediate stage 18is fed to the first stage 16 through the recirculation pump 24. In bothconfigurations, valve V9 is a control valve operated to control theconcentrate flow. A high cross-flow rate of 6-8 m3/h of concentrate perNF element was achieved in all stages.

In a second example, shown in FIG. 2, a second system 40 was designedfor treating an industrial effluent with over 200 mg/l of COD, 90 m3/hof permeate flow, and 95% of recovery. The second system 40 has fourstages 16, 18, 19 and 20. The first stage is divided into two portions,a first portion 16A and a second portion 16B. The recirculation pump 24and the order of flow can be switched between the last stage 20 and thefirst portion 16A of the first stage 16. When switched, the firstportion 16A of the first stage 16 receives concentrate last and the laststage 20 receives feed water first in parallel with the second portion16B of the first stage.

The feed pump 22 is a high pressure pump rated for 100 m3/h of outputand 110 m of head. The second system 40 also has a booster pump 46 ratedfor 40 m3/h of output and 30 m of head, and a recirculation pump 24rated for 56 m3/h of output and 40 m of head.

The NF system has 138 nominal 8 inch (20 cm) NF elements inserted in 23pressure vessels. Each pressure vessel holds 6 elements in series. Thefirst stage 16 has eight pressure vessels. Five of these pressurevessels are plumbed in parallel and make up the first portion 16A. Theremaining three pressure vessels are plumbed in parallel and make up thesecond portion 16B. A first intermediate stage 18 has six pressurevessels plumbed in parallel. A second intermediate stage 19 has threepressure vessels plumbed in parallel. The last stage 20 has fivepressure vessels plumed in parallel. 5 respectively. First portion 16Aand third stage 20 are identical to facilitate switching between them.Second portion 16B always receives feed water 12 directly from the feedpump 22.

When the valves are configured in a first configuration as shown in FIG.2, valves V1, V3, V5 and V7 are open and valves V2, V4, V6 and V8 areclosed. The feed water 12 is pumped to the first portion 16A and thesecond portion 16B of the first stage 16 by the feed pump 12.Concentrate from the first stage 16 is fed to the first intermediatestage 18 and then to the second intermediate stage through the boosterpump 46. Concentrate from the second intermediate stage 19 is fed to thelast stage 20 through the recirculation pump 24. When the valves areconfigured in a second configuration such that the first portion 16A andthe last stage 20 are switched, valves V2, V4, V6 and V8 are open andvalves V1, V3, V5 and V7 are closed. The feed water 12 is pumped to thelast stage 20 and the second portion 16B by the high pressure pump 22.The concentrate from the last stage 20 and the first portion 16B is fedto the first intermediate stage 18 and then to the second intermediatestage through the booster pump 46. Concentrate from the secondintermediate stage 19 is fed to the first portion 16A through therecirculation pump 24. Valve V9 is used as a control valve to controlthe concentrate flow in both configurations.

Optionally, though not shown in FIG. 2, the valves and conduits of thesecond system 40 could be changed such that at some times anotherselected portion of the first stage 16 receives the concentrate last andthe last stage 20 receives feed water first in parallel with theremainder of the first stage. However, it is desirable for the portionof the first stage 16 that is switched with the last stage 20 to havethe same number of elements and pressure vessels as the last stage 20such that the recirculation pump 24 and second system 40 as a wholeworks well in both configurations. In the second system 20, the firststage 16 does not have twice as many pressure vessels as the last stage20 as a result of optimizing the system design. While it would bepossible to rotate which five of the eight pressure vessels of the firststage 16 are switched with the last stage 20, this complication istypically not justified since the primary purpose of switching the orderof flow is to allow the last stage 20 a period of being exposed toun-concentrated feed water 12. If the first stage 16 happened to havetwice as many elements and pressure vessels 20, it would be easier torotate which portion of the first stage 16 is switched with the laststage 20. However, in most cases, the ability to switch the last stage20 with either portion 16A, 16B in the first stage 16 would not justifyaltering an optimized choice of the number of elements and pressurevessels in each stage. These comments assume that fouling in the laststage 20 can be adequately controlled by exposing the last stage 20 toun-concentrated feed water 12 for one half of the system operating timeor less. If not, then the system could be modified to allow for a methodof operation such that the pressure vessels of the first stage 16receive concentrate last more than half of the operating time, but asystem of rotation between them results in each individual pressurevessel receiving concentrate last for less than half of the operatingtime.

In a third example, shown in FIG. 3, a third system 42 was designed fortreating an industrial effluent with over 200 mg/l of COD, 90 m3/h ofpermeate flow, and 90% of recovery. A recirculation pump 24 was used forthe last stage 20. The third system 42 is similar to system 10 of FIG. 1but without valves and conduits allowing the order of the first stage 16and last stage 20 to be reversed.

The third system 42 has 126 nominal 8 inch (20 cm) NF elements insertedin 21 pressure vessels. Each pressure vessel holds 6 elements in series.A high pressure feed pump 22 is rated for 100 m3/h of output and 110 mof head. A recirculation pump 24 is rated for 110 m3/h of output and 40m of head. The thirds system 42 has three stages 16, 18, 20. The firststage 16 and last stage 20 each have 8 pressure vessels plumbed inparallel. The intermediate stage 18 has 5 pressure vessels plumbed inparallel. The feed water 12 is pumped to a feed inlet of the first stage16 by the high pressure pump 22. The concentrate from the first stage 16is fed to a feed inlet of the intermediate stage 18. Concentrate fromthe intermediate stage 18 is to a feed inlet of the third stage 20through the recirculation pump 24.

This written description uses examples to disclose the invention and toenable any person skilled in the art to practice the invention,including making and using any devices or systems and performing anyincorporated methods. The patentable scope of the invention is definedby the claims, and may include other examples that occur to thoseskilled in the art.

I claim:
 1. A process of treating a feed water comprising the steps of:flowing the feed water through a plurality of stages of membranefiltration with concentrate staging between the stages and concentraterecirculation in the last stage.
 2. The process of claim 1 furthercomprising the step of periodically flowing the feed water through theplurality of stages of membrane filtration with the order of flow andconcentrate recirculation switched between a portion of the first stageand the last stage.
 3. The process of claim 1 wherein the stagescomprise nominal 8 inch spiral wound modules and the feed or concentrateflow rate through each stage is at least 6 m³/h.
 4. The process of claim1 wherein the process is operated at a recovery rate of 80% or more. 5.The process of claim 1 wherein the feed water has at least 200 mg/L ofchemical oxygen demand.